In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as better control of contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
In CT systems, digital images are generated to provide a two dimensional view of an object. For example, utilizing a CT fluoroscopic system, the CT system may be used to provide images to help in guiding a medical treatment device to a desired location within a patient.
With known CT systems, the general objective is to provide the physician with as much useful information as quickly as possible to complete the procedure. In order to properly guide the device, the physician may require a real-time enlarged, or magnified, image. With known CT systems various methods are used for generating the real-time image, e.g., nearest neighbor and bilinear interpolation.
While some of these methods, e.g., nearest neighbor, are included in standard graphical packages, for example, Open GL, these methods produce poor quality images. However, those methods that produce high quality real-time images, e.g., bicubic interpolation, are not readily available for execution in a commonly available Open GL system.
It would be desirable to generate high quality real-time enlarged images from CT scan images. Particularly, it would be desirable to generate high quality real-time enlarged images utilizing commonly available image processing routines, for example Open GL. It would also be desirable to approximate bicubic interpolation using such routines. cl SUMMARY OF THE INVENTION
These and other objects may be attained by a system which includes apparatus and methods for altering the spatial characteristics of a digital image. The apparatus and methods are generally referred to herein as real-time image magnification. More particularly, in accordance with one embodiment of the present invention, after collecting a digital image, an enlarged image is generated by interpolating and filtering the original digital image data.
In one aspect, two pass linear interpolation and one dimensional filtering is utilized to generate the enlarged image. Particularly, the first pass performs interpolation in the x direction, and the second pass repeats the same procedure for interpolation in the y direction. The interpolations in the x and y directions are performed in two steps. Initially, linear interpolation is applied to the original digital image to generate interpolated data. A one dimensional convolution filter is then applied to the interpolated data. Utilizing this process, a high quality enlarged image may be generated for any integer zoom factor.
In another aspect, a two step procedure including interpolation and two dimensional filtering is utilized. Particularly, the first step performs bilinear interpolation of the digital image to generate interpolated data. The interpolated data is then filtered by a two dimensional convolution filter.
The above described algorithms generate high quality enlarged images from an original digital image. Further, the algorithms approximate bicubic interpolations using commonly available graphic packages.